RE: CRC32C Parallel Computation Optimization on ARM

On Date: Thu, 2 Nov 2023 09:35:50AM -0500, Nathan Bossart wrote:

>On Thu, Nov 02, 2023 at 06:17:20AM +0000, Xiang Gao wrote:
>> After reading the discussion, I understand that in order to avoid performance
>> regression in some instances, we need to try our best to avoid runtime checks.
> >I don't know if I understand it correctly.

>The idea is that we don't want to start forcing runtime checks on builds
>where we aren't already doing runtime checks. IOW if the compiler can use
>the ARMv8 CRC instructions with the default compiler flags, we should only
>use vmull_p64() if it can also be used with the default compiler flags. I
>suspect this limitation sounds worse than it actually is in practice. The
>vast majority of the buildfarm uses runtime checks, and at least some of
>the platforms that don't, such as the Apple M-series machines, seem to
>include +crypto by default.

>Of course, if a compiler picks up +crc but not +crypto in its defaults, we
>could lose the vmull_p64() optimization on that platform. But as noted in
>the other thread, we can revisit if these kinds of hypothetical situations
>become reality.

>> Could you please give me some suggestions about how to refine this patch?

>Of course. I think we'll ultimately want to independently check for the
>availability of the new instruction like we do for the other sets of
>intrinsics:
>
> PGAC_ARMV8_VMULL_INTRINSICS([])
> if test x"$pgac_armv8_vmull_intrinsics" != x"yes"; then
> PGAC_ARMV8_VMULL_INTRINSICS([-march=armv8-a+crypto])
> fi
>
>My current thinking is that we'll want to add USE_ARMV8_VMULL and
>USE_ARMV8_VMULL_WITH_RUNTIME_CHECK and use those to decide exactly what to
>compile. I'll admit I haven't fully thought through every detail yet, but
>I'm cautiously optimistic that we can avoid too much complexity in the
>autoconf/meson scripts.

Thank you so much!
This is the newest patch, I think the code for which crc algorithm to choose is a bit complicated. Maybe we can just use USE_ARMV8_VMULL only, and do runtime checks on the vmull_p64 instruction at all times. This will not affect the existing builds, because this is a new instruction and new logic. In addition, it can also reduce the complexity of the code.
Very much looking forward to receiving your suggestions, thank you!
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